Device-to-device (D2D) technology is attractive and obtaining considerable attention because of its ability to offer new services, improve system throughput, and offer a better user experience. There are two main identified D2D technologies: discovery and communication. In discovery, a user equipment (UE) attempts to discover neighboring UEs, either on its own or directed by the enhanced Node B (eNB). In communication, one UE directly communicates with another UE without the data transiting through the eNB. A D2D link is a direct communication between two UEs, without having the radio bearer for the communication transiting through the eNB.
There are two main ways of performing discovery: eNB-assisted discovery and open discovery. With eNB-assisted discovery, one UE is directed to transmit a signal (e.g., a sounding reference signal (SRS) signal), and another UE is required to listen and report the signal quality to the eNB. The eNB can, based on this reported signal quality, decide if proximity services (ProSe) can be enabled for these two UEs. With open discovery, any UE can transmit a “beacon” signal to advertise its presence to other UEs. Note that this process can possibly involve UEs in the idle state (idle UEs). In 3GPP, a UE can have logical operating states: idle and connected (radio resource control (RRC)-connected). 3GPP defines open discovery as a Type 1 discovery procedure where resources for discovery signal transmission are allocated on a non-UE-specific basis. eNB-assisted discovery is noted as a Type 2 discovery procedure where resources for discovery signal transmission are allocated on a per UE specific basis. Further, Type 2A discovery is where resources are allocated for each specific transmission instance of discovery signals while Type 2B discovery has resources semi-persistently allocated for discovery signal transmission.